Backport robustness improvements from python-s7comm

Backports concrete robustness improvements from nikteliy/python-s7comm
to the native Python S7 implementation:

- 210+ S7 protocol error codes from Wireshark's S7 dissector covering
  USERDATA parameter errors (0xD0xx), protocol errors (0x8xxx), and
  resource errors. parse_response() now raises descriptive S7ProtocolError
  on header errors, and USERDATA response parsing logs warnings.

- Stale packet detection with retry: new validate_pdu_reference()
  validates response sequence numbers. New _send_receive() helper wraps
  the send/receive/parse pattern with automatic retry on stale packets
  (up to 3 retries). Refactored ~15 call sites in client.py to use it.

- Automatic PDU splitting for read_area()/write_area(): requests
  exceeding the negotiated PDU length are automatically chunked,
  preventing PLC rejections on large reads (e.g. db_get() on big DBs).

- TPDUSize enum (ISO 8073) with configurable COTP negotiation: replaces
  hardcoded 0x0A with a proper IntEnum (128-8192 bytes).

- MAX_VARS=20 limit on read_multi_vars()/write_multi_vars(): raises
  ValueError when exceeding the S7 protocol limit.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: gijzelaerr